DHCP server exhausts pool while leases free

November 2008


In order to get some information across to our VoIP phones, the DHCP server serves them an IP umber for a few minutes, after which they connect in a way independent from the DHCP server. Lately, some VoIP phones don't get connected any more, as reflected by unknown client lines in the DHCP logs.


The unknown client messages to indicate that a client is unknown, which in DHCP parlance means that it doesn't have a host declaration. However, it seems that clients that don't have a host declaration (so the should get an unknonw client-line but are still going to get an address from a dnamic pool will not get an unknown-client line. So unknown client in the logs actually seems to serve as an indicator that a client is not going to be served.

/etc/dhcp3/dhcpd.conf Has been edited extensively lately, about a day prior to the first unknown clients. It now looks like:

shared-network MY-NET

#some options skipped for brevity"

set vendor-string = option vendor-class-identifier;
option NortelVoIPPhoneIdentifier code 128 = text;
option NortelVoIPVLANIdentifier code 191 = text;

class "VoIP" {
     match if option vendor-class-identifier = "Nortel-i2004-A";

subnet netmask


                allow members of "VoIP";
                deny known-clients;
                option routers,;
                option NortelVoIPVLANIdentifier "VLAN-A:824.";

                default-lease-time 180;
                max-lease-time 300;

#rest of file skipped